Programmed-temperature desorption

Ideally, the probe molecules used in the chemical characterization experiments should simulate as closely as possible the behaviour of the actual reactants. This is rarely possible and so simple probe molecules (H2, O2, CO, NO) are used instead. Some additional information about the nature of the surface may be obtained using thermal analysis methods, e.g., calorimetry, temperature-programmed desorption, temperature-programmed reaction. 

The adsorption of gaseous ammonia can therefore be used to measure the number of acid sites. Because adsorption of ammonia is more difficult to measure than desorption, temperature-programmed desorption of ammonia is often employed to measure the number of aetive sites of solid-acid catalysts. Integration of the thermal desorption plots gives the total number of acid sites and the desorption temperature indicates their strength. Ion-exchangers cannot, however, readily be heated to the temperatures required for complete desorption of ammonia without being decomposed. 

Ammonia adsorption-desorption + Temperature Programmed Desorption (TPD) runs [11] were performed in order to study the adsorption-desorption of NH3 onto the catalyst. Experiments were typically performed at a GHSV of 92,000 h by feeding 1,000 ppm of ammonia in the presence of 2 % O2 and 1 % H2O at constant adsorption temperature (between 200 and 400 °C) when the catalyst adsorption capacity was saturated, NH3 and O2 were shut off and a temperature ramp from 50 to 550 °C at 15 °C/min was started. Temperature Programmed Reaction (TPR) runs [11] were performed to study the gas-phase reactivity on increasing temperature the reactants were fed at constant temperature, then a temperature ramp at 2, 10, or 20 °C/min was run. TPR experiments were typically carried out in the presence of oxygen (2 %) and water vapor (1 %) with GHSV between 90,000 and 230,000 h the reactant feed concentration varied between 250 and 1,000 ppm. 

TPD Temperature-programmed desorption [171, 172] The surface is heated and chemisorbed species desorb at characteristic temperatures Characterization of surface sites and desorption kinetics... 

Temperature-programmed desorption (TPD) is amenable to simple kinetic analysis. The rate of desorption of a molecular species from a uniform surface is given by Eq. XVII-4, which may be put in the form... 

Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

The temperature-programmed desorption of NO2 shows a maximum rate at 330 K using a heating rate of 7.53°C/sec. Take A to be lO sec , and assume that Eq. XVIII-1 applies. (Note Ref. 344.)... 

TPD Temperature programmed desorption After pre-adsorption of gases on a surface, the desorption and/or reaction products are measured while the temperature Increases linearly with time. Coverages, kinetic parameters, reaction mechanism... 

Thenual desorption spectroscopy (TDS) or temperature progranuned desorption (TPD), as it is also called, is a simple and very popular teclmique in surface science. A sample covered with one or more adsorbate(s) is heated at a constant rate and the desorbing gases are detected with a mass spectrometer. If a reaction takes place diirmg the temperature ramp, one speaks of temperature programmed reaction spectroscopy (TPRS). 

Fig. 15. Temperature-programmed desorption (TPD) spectra SWCNTs, (b) activated carbon and (c) open-tip SWCNTs [52].

B. Meng, W. H. Weinberg. Monte Carlo simulation of temperature programmed desorption spectra. J Chem Phys 700 5280-1589, 1994. 

In a recent paper [11] this approach has been generalized to deal with reactions at surfaces, notably dissociation of molecules. A lattice gas model is employed for homonuclear molecules with both atoms and molecules present on the surface, also accounting for lateral interactions between all species. In a series of model calculations equilibrium properties, such as heats of adsorption, are discussed, and the role of dissociation disequilibrium on the time evolution of an adsorbate during temperature-programmed desorption is examined. This approach is adaptable to more complicated systems, provided the individual species remain in local equilibrium, allowing of course for dissociation and reaction disequilibria. 

This model system corresponds to the conditions under which flash desorption experiments are performed. The temperature programed desorption of Amenomyia and Cvetanovi6 is based on different model requirements as will be dealt with in Section IV.B. Therefore, the following treatment in the present section is pertinent only to the flash desorption conditions. 

Application of a Temperature-Programmed Desorption Technique to Catalyst Studies... 

Figure 2.27. Temperature programmed desorption (TPD) spectra of carbon monoxide (measured by Ap) as a function of temperature from nickel surfaces (a) Ni(l 11), (b) Ni(l 11) when the initially dosed surface has been subjected to an electron beam (150 pA for 10 minutes over an area of 1 mm2) and (c) a cleaved nickel surface.85 Reprinted with permission from Elsevier Science.

P. Berlowitz, C. Megiris, J.B. Butt, and H.H. Kung, Temperature-Programmed Desorption Study of Ethylene on a Clean, a H-Covered, and an O-Covered Pt( 111) Surface, Langmuir 1, 206-212 (1985). 

As already noted the strength of chemisorptive bonds can be varied in situ via electrochemical promotion. This is the essence of the NEMCA effect. Following initial studies of oxygen chemisorption on Ag at atmospheric pressure, using isothermal titration, which showed that negative potentials causes up to a six-fold decrease in the rate of 02 desorption,11 temperature programmed desorption (TPD) was first used to investigate NEMCA.29... 

S. Neophytides, D. Tsiplakides, and C.G. Vayenas, Temperature-Programmed Desorption of Oxygen from Pt-films Interfaced with Y203-Doped Zr02> /. Catal. 178, 414-428 (1998). 

---